In recent years, research and development have been actively continuing on optoelectronic integrated circuit (OEIC) devices for processing optical signals propagating in an optical waveguide. Particularly, attention is being paid to an optical waveguide device integrating an optical waveguide structure formed on a substrate and a photodetector for detecting the light propagating through such a waveguide.
Optical waveguide devices well known in the prior art can be divided into the evanescent-coupling type shown in FIG. 1 and the butt-coupling type shown in FIG. 2.
The evanescent-coupling type optical waveguide shown in FIG. 1 includes an n-type optical waveguide layer 10 formed on an n-type semiconductor substrate 12 with an intermediate n-type buffer layer 14. An intrinsic or n.sup.- -type light absorbing layer 16 and an overlying p-type contact layer 18 are formed over a length L of the optical waveguide layer 10. This light absorbing layer 16 and the contact layer 18 form a PIN photodiode structure. Moreover, a pair of opposing electrodes 20 and 22 are formed as shown in the figure and are used to apply a bias voltage to the PIN photodiode.
The incident light from an optical fiber 24 aligned with the optical waveguide device propagates within the optical waveguide layer 10 and is absorbed by the light absorbing layer 16, thereby generating an electrical signal across the electrodes 20 and 22.
This evanescent-coupling type device provides the advantage of forming the optical waveguide 10, the light absorbing layer 16 and the contact layer 18 on the semiconductor substrate 14 through an uninterrupted series of epitaxial growth steps. On the other hand, this device has a disadvantage that the coupling efficiency between a pair of layers is low and therefore the light absorbing efficiency becomes low because the light absorbing layer 16 directly overlies the optical waveguide layer 10.
A butt-coupling type device shown in FIG. 2 also includes an optical waveguide layer 26 and an intrinsic or n.sup.- -type light absorbing layer 28 both overlying the n-type buffer layer 12 formed on the semiconductor substrate 14. A p-type contact layer 32 forms a PIN photodiode together with the light absorbing layer 28. A device of this type shows very high light absorbing efficiency because a greater part of the light propagating through the optical waveguide layer 26 is absorbed by the light absorbing layer 28. However, to form the optical waveguide layer 26 and the horizontally adjacent light absorbing layer 28, one of the layers 26 or 28 is formed by epitaxial growth, and that layer 26 or 28 is then partially removed by etching. Then the other layer 28 or 26 must also be formed by epitaxial growth. That is, epitaxial growth is required twice with an intermediate etching step and thereby fabrication becomes correspondingly difficult.
The evanescent-coupling type device shown in FIG. 1 has a more practical structure than the butt-coupling device shown in FIG. 2 because it can be formed with only a single uninterrupted sequence of epitaxial growth. However, as described, this structure has a disadvantage that the light coupling efficiency is low and the light absorbing coefficient is also low. It is thus essential to elongate the length L of the light absorbing layer 16 for improvement of the light absorbing efficiency but thereby the parasitic capacitance of the PIN diode becomes larger, thus degrading high-speed operational characteristics.
One possible method of improving the light absorbing coefficient in the evanescent-coupling device while keeping short the length L of the light absorbing layer 16 is to improve the light coupling efficiency through a reduction of an internal waveguide ray angle (the angle .THETA. in FIG. 5, to be described later) by improving the light coupling efficiency. The angle reduction is achieved by making thinner the optical waveguide layer 10 and by making a larger difference of refractive indices between the buffer layer 14 or substrate 12 and the optical waveguide layer 10. However, a thinner optical waveguide layer 10 and a smaller difference of refractive indices between the buffer layer 14 or substrate 12 and the optical waveguide layer 10 are undesirable since the confinement of light in the optical waveguide layer 10 is thereby intensified and the optical coupling efficiency with the optical fiber 24 associated with the device is lowered. Namely, the light confinement region in the optical waveguide layer 10 becomes thereby much narrower in comparison with a core region 34 of the optical fiber 24. Also, the difficulty of input coupling by other means, such as lenses, is thereby rendered more difficult.
A thinner light absorbing layer 16 is a typical method of improving the light absorbing efficiency while maintaining a short length L for the light absorbing layer 16. But an excessively thin light absorbing layer 16 results in a larger parasitic capacitance of the PIN photodiode, thus degrading high-speed operational characteristics.
Koch et al have disclosed a type of an evanescent-coupling type of device having an anti-resonant reflecting optical waveguide (ARROW) in a technical article entitled "Wavelength selective interlayer directionally grating-coupled InP/InGaAsP waveguide photodetection" appearing in Applied Physics Letters, volume 51, 1987 at pages 1060-1062. As is shown in their FIG. 1, the light field distribution is usually confined to the waveguide layer by a cladding layer of anti-resonant thickness. However, the cladding layer is patterned to form a grating adjacent a semiconductor photodetector layer to thereby couple or scatter the otherwise guided light into the photodetector layer. The disadvantages of the Koch design is the necessity for regrowth over the grating and the very high wavelength sensitivity of the grating.
Kokubun et al have disclosed a related ARROW photodetector in a technical article entitled "Monolithic Integration of ARROW-type Demultiplexer and Photodetector" appearing in Proceedings of 14th European Conference on Optical Communications (ECOC '88), Brighton UK, September 1988 at pages 231-234. The Kokubun structure is illustrated in FIG. 3. An n-type region 80 is diffused into a p-type silicon substrate 82. A lower aluminum electrode 84 contacts the p-type substrate 82 and an upper aluminum electrode 86 contacts the n-type diffusion region 80 but is isolated from the substrate 82 by insulating layer 88. When the p-n junction between the n-type region 80 and the p-type substrate 82 is reversed biased, it forms a depletion region with a depletion edge 90, thereby forming an optical detector for an overlying waveguide structure 92. The waveguide structure includes a SiO.sub.2 waveguide core layer 94, an antiresonant cladding layer 96 adjacent the core layer 94, and a lower cladding layer 98. Further, an anti-reflection layer 100 is placed between the lower cladding layer 98 and the n-type diffusion region 80. The anti-reflection layer 100 has an index of refraction intermediate between those of the lower cladding layer 100 and the silicon n-type region 80. However, the thickness of Kokubun's lowest cladding layer 98 (.about.2 .mu.m) puts the photodetector beyond the tail of the optical intensity distribution of the evanescent wave in the waveguide layer 94. Therefore, Kokubun introduced a leakage structure in the waveguide structure 92 overlying the photodetector so as to couple the propagating light toward the photodetector. This work was later described in more detail by T. Baba et al in a technical article entitled "Monolithic Integration of an ARROW-Type Demultiplexer and Photodetector in the Shorter Wavelength Region" appearing in Journal of Lightwave Technology, volume 8, 1990, at pages 99-104. A disadvantage of this approach is that a complex buried structure is needed to define both the photodetector and the leakage structure. Furthermore, the light leakage is very wavelength sensitive. Also, the upper electrode 86 is formed over a side extension of the n-type diffusion region 80. The extra extended area of the diffused region produces an extra amount of parasitic capacitance 102 across the depletion region. The higher parasitic capacitance 102 increases the total photodetector capacitance and prevents high-speed operation of the photodetector.